要用排序顺序映射的Java列表[重复]

Question

public class Country {
    private String countryCode;
    private String countryName;

    
    public Country(String countryCode, String countryName) {
        super();
        this.countryCode = countryCode;
        this.countryName = countryName;
    }

    public String getCountryCode() {
        return countryCode;
    }

    public void setCountryCode(String countryCode) {
        this.countryCode = countryCode;
    }

    public String getCountryName() {
        return countryName;
    }

    public void setCountryName(String countryName) {
        this.countryName = countryName;
    }

    @Override
    public String toString() {
        return "Country [countryCode=" + countryCode + ", countryName=" + countryName + "]";
    }

}

逻辑

public class Java8_2_ListToMap {
    public static void main(String[] args) {
        
        Map<String, String>  countryMaps1 = getSortedCountryMap(getCountryLists());
        countryMaps1.forEach((k, v) -> System.out.println("Key is " + k + " Value is " + v));

    }

    private static List<Country> getCountryLists() {
        return Arrays.asList(new Country("IN", "India"), new Country("US", "United States"),
                new Country("GR", "Germany"), new Country("UK", "United Kingdom"));
    }



    private static Map<String, String> getSortedCountryMap(List<Country> countryLists) {
        
        
        Map<String, String> countryMap  = countryLists.stream()
                .sorted(Comparator.comparing(Country::getCountryCode))
                .collect(Collectors.toMap(Country::getCountryCode, Country::getCountryName));
        return countryMap;
    }
}

输出为

Key is IN Value is India
Key is UK Value is United Kingdom
Key is GR Value is Germany
Key is US Value is United States

Answer 1

默认情况下，collectors.tomap创建一个HashMap，该HashMap没有排序。有多个map实现，它们是有序的，可能适合您的用例。

例如，您可以选择使用treemap，它自动按自然顺序或按您选择的比较器对其键进行排序。

另一个选项是LinkedHashMap，它会记住键的插入顺序。

// TreeMap
Map<String, String> countryMap  = countryLists.stream()
        .sorted(Comparator.comparing(Country::getCountryCode))
        .collect(Collectors.toMap(Country::getCountryCode,Country::getCountryName, String::join, TreeMap::new));

// LinkedHashMap
Map<String, String> countryMap  = countryLists.stream()
        .sorted(Comparator.comparing(Country::getCountryCode))
        .collect(Collectors.toMap(Country::getCountryCode,Country::getCountryName, String::join, LinkedHashMap::new));